Generally, in a wire-cut electric discharge machining apparatus, an electric discharge energy is reduced to execute processing, in order to produce an superior surface finish. Here, the electric discharge energy is in proportion to a product between an electric discharge current Ip and an electric discharge on-time .tau.p. The electric discharge current Ip and the electric discharge on-time .tau.p in the case of electric discharge of a capacitor can be found by the following equations: ##EQU1## where Eo is electric discharge voltage;
Ea is arc voltage; PA1 C is an electrostatic capacity; and PA1 L is an inductance.
As described above, the wire-cut electric discharge machining apparatus has the following problem. That is, the discharge energy cannot be controlled to a value smaller than a certain value, because the machine body has a stray electrostatic capacity, so that there is a limit in to the quality of the surface finish of a processed surface.
In the conventional arrangement, in order to solve the above-discussed problem, there has been proposed in Japanese Patent Laid-Open No. SHO 61-197128 and Japanese Patent Laid-Open No. SHO 61-270022 a machine in which, in order to improve the surface finish during finish processing, a machine body and a workpiece are electrically isolated from each other. However, in general processing including rough processing and semi-finish operations conducted until the finish operation, the machine body and the workpiece are short-circuited to each other. These two conditions are automatically executed by the use of an electromagnetic switch.
FIG. 12 is a view showing a conventional wire-cut electric discharge machining apparatus. In FIG. 12, the reference numeral 1 denotes a machine body; 1a, a column; 1b, a bed; 2 and 3, cross tables movable respectively in an X-direction and a Y-direction, which are mounted on the bed lb; 4, a metal surface plate or a jig which is mounted on the cross table 3; 5, a wire electrode; 6, a supply bobbin for supplying the wire electrode 5; 7, a powder brake for applying a tension to the wire electrode 5; 8a, an upper guide for guiding the wire electrode 5 to a location between a pair of poles; 8b, a lower guide; 9, a pair of winding rollers for winding the used wire electrode 5; 10, the used wire electrode; 11, a processing electric power source; 12, a control unit; 13, a workpiece; 14a, a feeder wire through which an energy is supplied to the wire electrode 5 from the processing electric power source 11; and 14b, a feeder wire through which the energy is supplied to the workpiece.
Further, the reference numeral 15 denotes a finish-processing electric power source; 16a and 16b, feeder wires through which an energy is supplied from the finish-processing electric power source 15; and 17, an electromagnetic switch. FIG. 13 is an enlarged view of the environment surrounding the electromagnetic switch, 17. The reference numeral 18 denotes a case for protecting the electromagnetic switch 17 from a processing liquid; 19a and 19b, short-circuit wires for short-circuiting the 4 and the workpiece 13 to each other; 20, a signal line from the control unit 12 which controls turning-on and -off of the electromagnetic switch 17; and 21, an insulating jig formed of an electric insulating material. The arrangement is such that the insulating jig 21 is fixedly mounted on the jig 4 by screws or the like, the workpiece 13 is mounted on the insulating jig 21, and the feeder wire 16a from the finish-processing electric power source 15 is connected to the workpiece 13, whereby the finish-processing electric power source 15 is electrically isolated from the jig 4, that is, from the machine body 1. Furthermore, when the electromagnetic switch 17 is turned on by the control unit 12, the jig 4 and the workpiece 13 are electrically connected through the short-circuit wires 19a and 19b.
The conventional machine is constructed as described above. Accordingly, in the normal processing including rough processing and semi-finish processing, the electromagnetic switch 17 is brought to the ON-condition. Specifically, one of a pair of poles of the processing electric power source 11 is connected to the wire electrode 5 through the feeder wire 14a, and the other pole is connected to the workpiece 13 through the feeder wire 14b, the short-circuit wire 19a, the electromagnetic switch 17 and the short-circuit wire 19b, so that a processing energy is supplied to execute processing. Moreover, in the finish processing, the electromagnetic switch 17 is brought to an electrically isolated condition. Specifically, one of a pair of poles of the finish-processing electric power source 15 is connected to the wire electrode 5 through the feeder wire 16a, and the other pole is connected to the workpiece 13 through the feeder wire 16b. Further, since the short-circuit wires 19a and 19b are isolated from each other, a stray capacity of the wire electrode 5 and with respect to 4, that is, the machine body 1 is eliminated. Thus, processing in which the stray capacity is very small is made possible. As described above, if the machine is arranged such that the electromagnetic switch 17 is automatically turned on and off depending upon selection of the processing electric power source 11 or the finish-processing electric power source 15, the surface finish at finish processing is improved, and provision of the processing electric power source eliminates the necessity for changing the feeder wires during processing.
The above-described wire electric discharge machining apparatus is advantageous in the case where the processing energy is supplied from the finish-processing electric power source 15 to execute processing which improves the surface finish. Conversely, in the case where the processing energy is supplied from the processing electric power source 11 to execute normal or usual processing, connection from the processing, electric power source 11 to the workpiece 13 is through to a path from the feeder wire 14b the jig 4 the short-circuit wire 19a, the electromagnetic switch 17, the short-circuit wire 19b, and the workpiece 13. Generally, the cross-sectional area of the jig 4 is extremely large relative to the feeder wires, and it is considered that a value of an inductance from a mounting position of the feeder wire 14b on the jig 4 to a mounting position of the workpiece 13 is substantially constant on the jig 4. Accordingly, in the case where the isolating jig 21 is used, since the feeder wire from the finish-processing electric power source 15 to the workpiece 13 passes through the electromagnetic switch 17, the inductance largely increases dramatically because of an increase in the length of feeder wire and because of the fact that a circuit is brought to a loop configuration.
"Wire Electric Discharge Machine" disclosed in Japanese Patent Laid-Open No. SHO 62-188625 describes such a conventional device.
The conventional wire electric discharge machining apparatus constructed as described above has the following problems. That is, the increase in the inductance reduces the processing current value, and reduces the processing speed or rate. Further, adhering of an electrode material of the wire electrode to the workpiece also increases. Thus, the processing treatment will be impeded or hindered. Furthermore, since the value of the processing current varies depending upon the length of the feeder wire, there is a problem if the length of the feeder wire can not be constant to produce a constant processing accuracy.
In view of the above, in the conventional wire discharge machine, the mounting position of the electromagnetic switch and the mounting position of the insulating jig must be as close as possible in order to reduce the inductance of the feeder wire by shortening the feeder wire. However, it is not practical that the electromagnetic switch 17 is mounted within a processing liquid, particularly, in a wire electric discharge machining apparatus of type in which a processed or processing section is immersed within the processing liquid. Accordingly, that the electromagnetic switch 17 must be mounted on a location spaced apart from the jig. Further, the mounting position of the insulating jig, that is, the mounting position of the workpiece can not be a position where the feeder wire can reach. Thus, the mounting position is limited or restricted.